1. Field of the Invention
This invention relates to a method for growth of a crystal, which is used for, for example, electronic devices such as semiconductor integrated circuits, optical integrated circuits, magnetic circuits, etc., optical devices, magnetic devices, piezoelectric devices, or surface acoustic devices, etc.
2. Related Background Art
In the prior art, a monocrystal of thin film to be used for semiconductor electronic devices or optical devices has been formed by epitaxial growth on a monocrystal substrate. For example, on a Si monocrystal substrate (silicon wafer), Si, Ge, GaAs, etc. have been known to be epitaxially grown from liquid phase, gas phase or solid phase, and also on a GaAs monocrystal substrate, a monocrystal such as GaAs, GaAlAs, etc. has been known to be epitaxially grown. By use of the semiconductor thin film thus formed, semiconductor devices and integrated circuits, emission devices such as semiconductor laser or LED, etc. are prepared.
Also, recently, ultra-high speed transistors by use of two-dimensional electron gas, ultra-lattice device, utilizing quantum well, etc. have been aggressively studied and developed. What has made these techniques possible is the high precision epitaxial technique such as MBE (molecular beam epitaxy by use of ultra-high vacuum) or MOCVD (metalloorganic chemical vapor depositon), etc.
In such epitaxial growth on a monocrystal substrate, it is necessary to adjust the lattice constants and coefficients of thermal expansion between the monocrystal material of the substrate and the epitaxial growth layer. If this adjustment is insufficient, lattice defects will be developed in the epitaxial layer. Also, the element constituting the substrate may be sometimes diffused into the epitaxial layer.
Thus, the method for forming a thin film monocrystal of the prior art by epitaxial growth can be understood to be greatly dependent on the substrate material. Mathews et al examined various combinations of substrate material and epitaxial growth layers (EPITAXIAL GROWTH, Academic Press, New York, 1975 ed. by J. W. Mathews).
The size of the substrate is presently about 6 inches in the case of Si wafer, and enlargement of the GaAs, sapphire substrate has been further delayed. In addition, since a monocrystal substrate is higher in cost, the cost per chip becomes higher.
Thus, to provide monocrystal growth capable of preparing a device of good quality, there has existed the problem that the kinds of appropriate substrate materials have been limited to a very narrow range.
On the other hand, studies and developments have been abundantly made for three-dimensional integrated circuit for accomplishing high integration and multi-function by laminating semiconductor devices in the direction normal to the substrate. Also, studies and developments have been actively done in recent years regarding large area semiconductor devices such as the solar battery, the switching transistor of liquid crystal picture element, etc. in which devices are arranged in an array on an inexpensive glass.
What is common to both of these products is that it is necessary to have a technique of forming a thin film monocrystal of a semiconductor material on an amorphous insulating material and forming an electronic device such as transistor, etc. thereon. Among them, it has been particularly desired to have a technique of forming a monocrystal semiconductor of high quality on an amorphous insulating material.
Generally speaking, when a thin film is deposited on a substrate of an amorphous insulating material such as SiO.sub.2, etc., due to deficiency of long distance order of the substrate material, the deposited film will have a crystalline structure which is amorphous or polycrystalline. Here, the amorphous film is a film in which the short distance order to the extent of the nearest atom may be maintained, but there is no longer distance order. A polycrystalline film is a film in which monocrystal grains having no specific crystal orientation are gathered together as separated through grain boundaries.
For example, when Si monocrystals are to be formed on SiO.sub.2 by the CVD method, if the deposition temperature is about 600.degree. C. or lower, they will become amorphous silicon, and at higher temperatures they will be polycrystalline silicons with grain sizes distributed between some hundred to some thousand angstroms. However, the particle sizes of polycrystalline silicon and distribution thereof will vary greatly depending on the method for formation.
Further, by melting and solidifying amorphous or polycrystalline film with an energy beam such as laser, rod-shaped heater, etc., a polycrystalline film with larger grain size of about micron to millimeter has been obtained (monocrystal silicon on non-single-crystal insulators, Journal of Crystal Growth, Vol. 63, No. 3, October, 1983 edited by G. W. Cullen).
On the thin film of the respective crystal structures thus formed, transistors were formed, and from their characteristics, electron mobilities were measured and compared with that of monocrystalline silicon. As the result, the polycrystalline silicon with a large grain size of some tim to some mm obtained by melting solidification is found to have a mobility to the same extent as in the case of monocrystal silicon, while the polycrystalline silicon having a grain size distribution of some hundred to some thousand angstroms is seen to have a mobility which is about 10.sup.-3 of that in the case of monocrystalline silicon. In the case of amorphous silicon, an electron mobility to the extent of about 2.times.10.sup.-4 of that in the case of monocrystalline silicon is obtained.
From these results, it can be understood that there is a great difference in electrical characteristics between the device formed in the monocrystal domain within crystal grains and the device formed as bridging over the grain boundary. That is, the deposited film on the amorphous material obtained by the prior art method will become an amorphous structure or a polycrystalline structure having a grain size distribution, and the device prepared thereon becomes greatly deteriorated in its performance as compared with the device prepared on the monocrystal layer. For this reason, the uses are limited to simple switching devices, solar battery, photoelectric transducing device, etc.
Also, the method for forming a polycrystalline thin film with greater grain size by melting and solidification has involved the problems such that enormous time is required for enlargement of grain size, that bulk productivity is poor and also that it is not fitted for enlargement of area, because amorphous or monocrystal thin film is scanned with energy beam for every wafer.
As described above, according to the method for forming crystals of the prior art, three-dimensional integration or enlargement of the deposited area could only be done with difficulty, could only be practically applied to devices with difficulty, such that it was impossible to form a monocrystal required for preparation of the device having excellent characteristics, easily and at low cost.